INTERACTION OF THE det (DETERMINATE) MUTANT WITH
OTHER FLOWERING GENES

Murfet, I.C.Department of Plant Science, University of
Tasmania

Hobart, Tasmania 7001, Australia

The recessive gene det
(determinate) causes the shoot to terminate in a flower after the
formation of a small number of reproductive nodes (1,2,4,5). Whether the
det mutant is strictly determinate in the botanical sense requires
clarification but the fact that the terminal flower is frequently offset
from the vertical (Fig. 1 ) suggests it may have arisen from an axial
flower primordium rather than by direct conversion of the apical meristem
itself.

The present study examined the
interaction between det and several of the other flowering genes in
Pisum. The det mutant was received from Dr Peter Matthews of
the John Innes Institute in the form of line JI 1358 which is tall
(Le) with a late flowering habit indicative of genotype LfSnDne (see 3). Most JI 1358 plants showed a very large
response to photoperiod indicating the presence of gene Hr but some
displayed only a limited quantitative response to photoperiod indicating
genotype hr. Thus JI 1358 may be heterogeneous for the
Hr-hr gene pair. Line JI 1358 was crossed with Hobart line
69, which is a very early flowering, day neutral dwarf with genotype
lfaEsnDnehrle,
and 96 F2 plants and several F3 progenies from
lfa or Lfsndet F2 plants were raised in 9 h short day
conditions (day 23°C, night 16°C).

The results of cross 69 x 1358 gave
no indication that det altered the effect of gene pairs
Lf-lfa, Sn-sn or
Hr-hr on node of flower initiation. For example, in the
F2 node of flower initiation ranged from 6-48 for Det
segregates and 7-52 for det segregates. However, the results of
this small study are not such that they would expose with certainty small
quantitative effects of det on node of flower initiation.
Nevertheless, the results did provide a very clear answer on several
points. For example, segregation for the Lf-lfa gene
pair was clearly apparent in both sndet (Fig. 2) and
Sndet plants on the basis of node of flower initiation.
Likewise segregation of the Sn-sn gene pair was entirely clear on
an lfa det background on the basis of time to first open
flower (Fig. 2) and several other traits such as flower bud abortion,
peduncle length and branching pattern. For example, lfa sn
det segregates produced few if any lateral branches while
lfa Sn det segregates produced a massive outgrowth of
laterals (Fig. 1) and peduncle length was much longer in the Sn
plants (Fig. 1 ).

Termination of mainshoot growth in
det plants produced several marked effects. With an
lfa Sn background det plants showed a more
precocious and profuse outgrowth of lateral branches than Det
plants (Fig. 1). In lfa Sn det plants these lateral
branches arose from both basal nodes (e.g. nodes 1 and 2) as well as
aerial nodes further up the stem. In Lfsn plants gene
let likewise resulted in the outgrowth of lateral branches but in
this case the outgrowth occurred only from aerial nodes, particularly
those just below the first flower (Fig. 1). In contrast, Lfsn
Det plants were wholly devoid of lateral branches (Fig. 1). Finally,
with an lfa Sn background det brought forward the
time of first open flower by, on average, 4 days compared with that of
Det segregates. This effect was significant at the 0.01 level. The
earlier development of the flower buds on the det plants appears to
have resulted from the availability of nutrients which in Det segregates
were diverted toward the growth of new vegetative organs. Indeed, the
forced growth in lfaSndet plants meant
the terminal flower in

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45

Fig. 1. The characteristics of five
individual, 60-day-oLd, tall plants derived from cross 69 x 1358 are used to illustrate some of
the differences between det and Det plants. Peduncles and
lateral branches are drawn to scale but internodes are shown on a fixed
module. Figures above a flower indicate the number of days from sowing to
opening of this flower and the flower/leaf relativity (see 3), e.g..for the DetIfa
Sn plant the flower at node 11 opened on day 47 and there
were just over 3 expanded leaves above the flower at that time. The main
shoot of the DetIfaSn plant was still
elongating actively at 60 days but the main shoot of the remaining plants
had ceased growth at this time. Photoperiod 9 h. The cotyledonary node is
counted as node 0.

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PNL Volume 21 1989 RESEARCH REPORTS

Fig. 2. Node of flower initiation
and days to first open flower for 32 tall det plants from several
F3 families of cross 69 x 1358. Note these are combined data
and illustrate distributions not ratios.

some cases opened one day ahead of
the penultimate flower and peduncle length reached a massive 26 cm in some
tall lfaSndet segregates (Fig. 1). The
several effects of the det gene detailed in the last paragraph all
appear to be correlative phenomena resulting from the loss of hormonal
influence of the apical meristem and bud, and the diversion of nutrients
and assimilates no longer required for continued growth of the main shoot.
One might expect that excision of the apical bud from normal (Det)
plants would produce the same result. In contrast, the gene
lfa had a totally unexpected effect on the expression of
det. Lfdet segregates, whether they flowered at node
10 or node 52, produced either one or at most two leaves subtending a
flower (Fig. 3). In contrast, lfadet segregates
produced between 3 and 6 leaves with axillary flowers before terminating
(Fig. 3). Put another way, det plants always produced at least 9 normal
leaves on the main shoot (2 scale leaves + 7 foliage leaves) while some
lfa plants initiated flowers as early as node 5. Thus
while expression of det is triggered by the onset of flower
initiation, the interval between the onset of flower initiation and the
termination of meristem activity is markedly influenced by the genotype at
the Lf locus and is clearly longer in an apex with genotype
Ifa than one with genotype Lf. Cross 69 x 1358
appears to be homozygous for gene E and it remains to be seen
whether this relationship also holds with background e since
lfaeSnDne plants are capable of
commencing flower initiation above node 9 (3). The expression of det in apices with alleles
lf or Lfd also remains to be
examined.

Expression of det in lateral
shoots tended to follow a similar pattern to that displayed by the main
shoot, i.e., the number of leaves with axillary flowers was about the same
as in the main shoot and very much dependent on the genotype at the
Lf locus. In contrast, the genotype at the Sn locus had
little or no effect on the expression of det and Lfsn plants, which commenced flowering at nodes 10-13, displayed a
near identical pattern to LfSn plants in which the node of
flower initiation ranged from 13-52 (Fig. 3). There is perhaps a slight
effect in lfa plants resulting from the 9 leaf limit for
the expression of det and the slightly lower flowering node of
lfasncompared with lfaSn plants (Fig. 3).

Some lfasndet plants produced pods at 5 or even 6 nodes on the main shoot.
This reproductive load is similar to that sustained by comparable
Det segregates. In such cases the lateral branching characteristic
of det plants failed to occur. In contrast, with Lfsndet segregates the small reproductive load on the main shoot was considerably less than
that sustained by comparable LfsnDet segregates and
vigorous outgrowth of aeriallaterals occurred in the
det plants while no laterals arose from their Det
counterparts.

The det gene provides an
opportunity to restructure the reproductive architecture of the pea plant
but correlative changes to other characters such as branching pattern and
peduncle length will need to be taken into
consideration.